Various mechanisms have been developed to reduce NOx emissions from lean-burning engines. One mechanism uses a catalyst known as a NOx trap. The NOx trap is a catalytic device typically positioned downstream of another catalytic converter in an emissions system, and is configured to retain NOx when the engine is running a lean air/fuel mixture for eventual reduction when the engine runs a more fuel rich air/fuel mixture. A typical NOx trap includes an alkali or alkaline metal, such as potassium or barium, which adsorbs NOx when the engine is running a lean air/fuel mixture. The engine can then be configured to periodically run a richer air/fuel mixture to produce carbon monoxide, hydrogen gas and various hydrocarbons to reduce the NOx in the trap, thus decreasing NOx emissions and regenerating the trap.
The use of a NOx trap can substantially reduce NOx emissions from a lean-burning engine with either spark or compression ignition. However, NOx traps are also susceptible to poisoning from sulfur in fuels, which may adsorb to the NOx adsorption sites in the form of sulfate (SO42-) or other adsorbed or stored sulfur compounds. Stored sulfur may prevent NOx from adsorbing to trap surfaces, thereby impeding proper trap performance.
Various methods of desulfating NOx traps may be used. In general, these methods involve heating the NOx trap to a temperature sufficient to allow the reduction of stored sulfur, and then producing a rich exhaust to reduce it to sulfur dioxide (SO2) or hydrogen disulfide (H2S). A rich/lean oscillation may be used during desulfation to help reduce hydrogen disulfide emissions. However, heating the trap to the temperatures used for desulfation and maintaining the elevated temperatures may cause the NOx absorption material in the trap to coarsen and degrade. This process may be referred to as thermal aging, and may degrade trap performance.
The inventors herein have realized that desulfation of a catalytic device may be more efficiently performed by operating the engine for a first interval in such a manner as to store and reduce NOx emissions in the catalytic device, thereby accumulating stored SOx in the catalytic device, and operating the engine for a second interval in such a manner as to remove a portion of the stored sulfur from the catalytic device while leaving a substantial portion of stored sulfur in the catalytic device. Such a procedure may provide for good NOx trap performance while reducing thermal aging caused by desulfation, as the rate of sulfur removal is highest during the initial portions of a desulfation process and decreases as the desulfation process progresses. Therefore, such a procedure may be used to remove sufficient sulfur for proper trap performance while reducing an amount of time the trap is subjected to potentially damaging desulfation temperatures.